1. Field of the Invention
The invention relates to a process for isolating organic compounds and/or separating these from each other in particular from aqueous solutions, after adsorption on inorganic solids, by cross-flow filtration.
2. Prior Art
Solutions for which the process according to the invention may be used arise, for instance, from the industrial synthesis of oligopeptides, in which sometimes not inconsiderable residual concentrations of the amino acids used as starting materials always remain in solution, together with the desired end product. A critical step in the production of pure peptides is isolation of the starting materials. Isolation of amino acids from dipeptides is especially difficult if processes of preparation are used in which the protective group required to protect the amino group is removed directly after coupling (e.g. during peptide coupling with N-carboxylic anhydrides) and a second unprotected amino acid is used for coupling. The free amino acids and free peptides present in solution in this case often have very similar pKi values and therefore very similar dissolution behaviour. Purification by crystallisation is therefore often impossible, or only possible by incurring large losses. A variety of chromatographic processes from the prior art is used for this purpose.
In the case of partition chromatography, the different partition equilibria of amino acids and peptides between two different solvent systems (aqueous and organic) is used. If these partition equilibria are not sufficiently far apart, purification by this method is difficult to impossible.
Affinity chromatography, in which differences in bonding strengths to specific reaction partners is used, is only suitable for very small concentrations.
Chromatographic processes which depend on hydrophobic interactions between a support and the substances to be purified (as are described for peptides and proteins in the PCT (SE 93/00582)) make use of the dependence of these interactions, and thus bonding to the support material, on the salt concentration in the elution medium. In these processes, the addition of non-polar, organic substances is often required in order to achieve sufficient separating power.
In comparison to the processes mentioned so far, ion-exchange chromatography is the most important with regard to industrial processes. The separation of amino acids and peptides here depends on differences in the isoelectric points of the substances. Amino acids and peptides are bonded in their cationic form by ion exchange on a cation exchanger at an acid pH. Separation is performed by elution with an increasing pH gradient in the elution medium. The individual amino acids and peptides are released and eluted at the pH which corresponds to their own isoelectric point.
All the chromatographic processes mentioned have the disadvantage that several stages are required to produce sufficient purity in the case of substances with similar and comparable functional groups. This means that several purification cycles, in which relatively large losses of valuable substance cannot be avoided, are required. In addition, the salt concentration in the solutions is sometimes increased quite considerably when eluting with the addition of salts or using a shift in pH. These salts, and possibly other additives which are required, then have to be removed again in a complicated procedure.
A further process, in which separation and purification is achieved using differences in the molecular weight and dimensions of the molecules being separated, is gel filtration. Here, a porous matrix which is generally based on organic substances is used. Larger molecules cannot diffuse into the pores and are rapidly eluted, while smaller molecules are retained. The disadvantage here, however, is the high pressure which has to be applied in order to obtain an adequate throughput.
The so-called cross-flow microfiltration process is also disclosed in the prior art, this being used, for instance, to purify beer from yeasts and solids which cause turbidity (DE-OS 4401 456).
In this process, the unclarified or pre-clarified beer flows over a porous membrane in a filter module, so that a transmembrane pressure is set up between the concentrated liquid flowing over the membrane and the filtrate on the other side. Some of the beer flowing over the membrane passes through it in a transverse direction and is collected as purified filtrate on the filtrate side. During the filtration process, the material being filtered out is sometimes deposited on the surface of the membrane as covering layer. This layer must not become too compact if passage through the membrane is to be guaranteed during the entire filtration process. In order to ensure this, the filtration process is interrupted at intervals and the covering layer on the membrane is partially dissolved using an alkaline solution.
This is one possibility of countering the unavoidable formation of covering layers on the filter itself. (Ullmann's Encyclopedia of Industrial Chemistry, vol. B2, Unit Operations 1, 5th ed. p. 10-21).